Light-Emitting Diodes (LEDs) integration is of major concern in the recent years for applications like signalling or for domestic lighting. Indeed, LEDs and more specifically high-brightness LEDs are expected to replace conventional lamps in lighting applications within few years.
Generally, these high-brightness LEDs have been developed with InGaN (gallium-indium) based materials grown on sapphire substrates that is an insulating material. The use of such substrate produces a high sensibility of the high-brightness LEDs to the electrostatic discharges.
Electrostatic discharges and electrostatic damages can occur at any point from manufacture to field service. It results from handling the devices in uncontrolled surroundings with inadequate ESD control practices. For instance, a forward biased ESD pulse will pass through the LED without damage, but a reverse biased ESD pulse will generate catastrophic failure.
InGaN LEDs dies are generally considered “Class 1” devices according to Military Standard Electrostatic Discharge Control (mil-std-1686c). To be considered “Class 1” the component needs survive to a voltage bias equal to 20 V, and to a voltage equal to 130 V from Human Body Model Testing. In order to avoid any reliability issue due to ESD discharges, LEDs manufacturers have many electronic devices available. The most popular ones are ceramic capacitors, Zener diodes, transient voltage suppression (TVS) diodes and Schottky diodes.
Among these devices, Zener diodes are widely used by design engineers worldwide for their low cost of fabrication. Further, they are more efficient than ceramic capacitors because they provide a stronger defense against overshoot voltage. Furthermore, they present also a higher clamping ratio (ratio between impulse clamping voltage and DC breakdown voltage), and they present a slower heat dissipation of ESD, which increases the clamping voltage level.
A correctly rated Zener diode reverse biased in parallel with the LED will allow voltage spikes to pass through the circuit in both directions without damaging the LED. Addition of a capacitor to smooth input signal is an appropriate corrective action to prevent electrostatic overstress (EOS) failures. To do that, it is common to use a Zener diode reverse biased in parallel with the LED on a ceramic carrier in order to have reliable lighting source.
Some LED manufacturers prefer to have back to back Zener diodes for ESD protection in order to have a symmetric device and to be able to measure the leakage current of the LED after assembly on the submount with the Zener diodes. A drawback of this solution is that it increases complexity and cost of the LED component. In a very competitive environment, it is an issue as the price of the LED has to decrease. So a very high pressure on ESD diode protection price is put.
In order to reduce the ESD protection price, the Zener diode has to be smaller and smaller while keeping the same ESD robustness. To overcome this issue, US 2007/0145411 proposes a method of manufacturing a trench polysilicon diode. The method includes forming a N− (P−) type epitaxial region on a N+ (P+) type substrate and forming a trench in the N− (P−) type epitaxial region. The method further includes forming an insulating layer in the trench and filling the trench with polysilicon forming a top surface of the trench. The method further includes forming P+ (N+) type doped polysilicon region and N+ (P+) type doped polysilicon region in the trench and forming a diode in the trench wherein a portion of the diode is lower than the top surface of the trench. This structure can be done by a better use of the piece of silicon available in 1 mm2. The trench diodes have the advantage to use all the volume of silicon and to present a huge P/N junction area.